Journal of Al-Nahrain University Vol.20 (2), June, 2017, pp.71-76 Science

71

Study the Structure, Morphology and Vibration Modes for

K2CrO4 and K2Cr2O7

Hasanain Saad Azeez and Mohammad R. Mohammad

Department of Applied Sciences, University of Technology, Baghdad-Iraq.

Abstract

In this work, the X-ray diffraction (XRD) measurement have been done to study the structure of

K2CrO4, and K2Cr2O7 powders.

The morphology of K2CrO4, and K2Cr2O7 thin films has been studied by using Atomic Force

Microscopy (AFM) technique.

Modes of vibrations for both K2CrO4, and K2Cr2O7 compounds were measured and discussed using

Fourier transform infrared spectroscopy (FTIR). [DOI: 10.22401/JNUS.20.2.09]

Keywords: structural, morphological, vibrational.

1. Introduction

1.1 Potassium chromate K2CrO4:

It is an inorganic solid compound, which

has a yellow color for the potassium salt of

chromateanion. It is known as a laboratory

chemical material, whereas sodium chromate

is an important in industrial material [1][2].

1.2 Potassium dichromate K2Cr2O7:

Potassium dichromate is one of the

crystalline inorganic chemical reagent.

Hexavalent chromium compounds are

harmful to health. K2Cr2O7 is widely used in

laboratories and industry as an oxidizing agent

because it is not deliquescent. Potassium

dichromate looks very bright and red-orange

color [3][4].

Table (1) shows the physical and chemical

properties of both K2CrO4 and K2Cr2O7

compounds.

Table (1)

Shows physical and chemical properties for K2CrO4, K2Cr2O7 [1-4].

physical and chemical properties Potassium chromate Potassium dichromate

Molecular Formula K2CrO4 K2Cr2O7

Molecular Weight 194.19 g/mol 294.18 g/mol

Physical State Poly crystalline powder Poly crystalline powder

Appearance Yellow coloured powder Orange coloured powder

PH (5%, 20 0C)

8.6 – 9.8 3.7 - 4

Density 2.73 g/cm3

2.676 g/cm3

Melting Point 968 0C 398

0C

Boiling Point 1000 0C 500

0C

Solubility in Water (200C) 629 g/L 125 g/L

Solubility in Alcohol Insoluble Insoluble

Refractive Index (nD) 1.74 1.738

2. Expermantal procedure

2.1 Prepartion of K2CrO4, K2Cr2O7 thin

films:

2.1.1 Substrate preparation:

For spin coating (K2CrO4) or (K2Cr2O7),

thin films have used glass substrates. At the

beginning, the substrates have been cleaned by

diluted hydrochloric acid (HCl), ethanol and

distilled water, then using electrical oven

supplied by (Jard) at temperature (80 C0) to

dry the substrate.

2.1.2 Film spin coating:

Potassium chromate powder or potassium

dichromate powder is grinded with grinder to

get fine powder then the ethanol is add to fin

Hasanain Saad Azeez

72

powder and enter the precipitate solution into

the ultrasonic vibration for (15) minutes to

become homogeneous solution. Spin coating is

applied by adding a drops of homogeneous

solution by Pipette to glass slide to get thin

film.

The summary of spin coating conditions is

showed in Table (2).

Table (2)

Summary of spin coating conditions.

parameters values

Speed 3000 (rpm)

Time 15 sec

acclr 5 sec

Temperature 25℃

3. Results and discussions

3.1 X- ray diffraction (XRD):

K2CrO4 powder is grinded with grinder to

get fine powder in order to get perfect

crystallinity of material as clarified in

Fig.(1A). The characteristics peaks of X-ray

diffraction pattern for K2CrO4 come at

2θ=29.0750o, 29.9389

o, 20.8283

o which

correspond to miller indices (211),( 031),(111)

respectively; according to the JCPDS

no.15-365 data. And all the hexagonal phase

(Orthorhombic structure) of K2CrO4 is

appeared in all diffraction peaks. The high

purity of K2CrO4 compound indicated

disappearance peak of impurity.

The lattice spacing (d) could be calculated

by using Bragg’s law; to calculate (d) lattice

spacing where ni=1.

2d sin = ni λ .............................................. (1)

where λ of the X-Ray wavelength is

(1.54 A0), is the diffraction angle in degree

and ni =1.

Also the grain size (D) can be calculated by

using Scherrer equation:

D=

...................................................... (2)

where k= is a constant (0.89<k<1),

β=FWHM. (full width at half maximum) of

the diffraction peak.

The particle size (D) is calculated using

equation (2).The average crystallite size D has

been calculated from equation (2) to be

12.95 nm.

K2Cr2O7 powder is grinded with

grinder to get fine powder in order to

get good crystallinity of material as shown in

Fig. (1B). The X-ray diffraction pattern

shows characteristic peaks of K2Cr2O7 at

2θ=27.1046o, 25.7088

o, 24.4468

o which

correspond to miller indices (2ī1),(210),(0ī2)

respectively according to the JCPDS no.0.27-

0380 data. All the diffraction peaks refer to

hexagonal phase (Anorthic Triclinic structure)

of K2Cr2O7 peaks of impurity which do not be

observed, that refers to the high purity of

K2Cr2O7 compound.

The particle size calculated using equation

(2) shows an average crystallite size D of

19.91 nm.

Table (3) shows XRD details for K2CrO4,

K2Cr2O7 powders.

Journal of Al-Nahrain University Vol.20 (2), June, 2017, pp.71-76 Science

73

Table (3)

XRD for K2CrO4 and K2Cr2O7 powder.

Material 2θ

(deg.)

Plane

(hkl)

FWHM

(deg.)

Grain size

(nm)

d

(Aº)

K2CrO4

K2Cr2O7

29.0750o

29.9389o

20.8283o

27.1046o

25.7088o

24.4468o

211

031

111

2ī1

210

0ī2

0.26980

0.36830

0.32350

0.24850

0.25470

0.14900

15.21

11.168

12.489

16.448

16.002

27.287

3.06876

2.98215

4.26140

3.28720

3.46242

3.63823

Fig.(1): The XRD spectra of the (A) K2CrO4, (B) K2Cr2O7 powders.

3.2 Atomic Force Microscopy (AFM):

In sensing devices application the

morphology plays an important role to the

physical and optical properties. Fig.(2 A)

refers to the 3D, 2D AFM images of the

K2CrO4 thin film, while Fig.(2 B) refers to the

3D, 2D AFM images of the K2Cr2O7 thin

film.

(A)

dif

frac

tion i

nte

nsi

ty (

a.u.)

2θ

Hasanain Saad Azeez

74

(B)

Fig.(2): Atomic force microscopy images at 3D and 2D of the (A) K2CrO4 thin film, (B) K2Cr2O7

thin film.

Fig.(3) refers to the granularity cumulation

distribution chart of the K2CrO4, K2Cr2O7 thin

films.

The calculated values of average grain size

and roughness are shown in Table (4).

(A)

(B)

Fig.(3): Granularity cumulation. distribution chart of the (A) K2CrO4 and (B)K2Cr2O7 thin films.

Journal of Al-Nahrain University Vol.20 (2), June, 2017, pp.71-76 Science

75

Table (4)

The value of average grain size and roughness.

Material Average. grain size (nm) Roughness(nm) Root,, Mean Square (nm)

K2CrO4 81.23 3.34 3.96

K2Cr2O7 75.53 0.815 0.974

3.3 Fourier Transform Infrared

Spectroscopy (FTIR):

Fig.(4A) shows the FTIR spectrum of

K2CrO4 powder. The CrO4-2

ion has tetrahedral

structure belonging to Td point group. This ion

has four fundamental bands; υ1 (symmetric

stretching band), υ2 (anti- symmetric stretching

band), υ3 (symmetric bending band), υ4 (anti-

symmetric bending band). The υ1 and υ2

vibrations are Raman active only (this is

because there are only changes in magnitude

or direction of polariziblity), while υ3 and υ4

vibrations are infrared active only (this is

because there are changes in magnitude of

dipole moment).

From the previous studies that a free

chromate ion as a result of its Td symmetry has

four normal modes of vibration represented by

A1,E1,T1 and T2 with frequencies υ1(847 cm-1

),

υ2 (348 cm-1

), υ3 (884 cm-1

), and υ4 (368 cm-1

)

respectively [5][6].

Fig.(4B) shows the FTIR spectrum of

K2Cr2O7 powder. The weak peak of K2Cr2O7

at 555.52 cm-1

is due to Cr-O-Cr symmetric

stretching vibration, while the very strong

peak at 754.19 cm-1

is due to Cr-O-Cr anti

symmetric stretching vibration, the medium

strong peak at 891.14 cm-1

is due to Cr-O3

symmetric stretching vibration (ms) and the

very strong peak at 935.51 cm-1

is due to Cr-O3

anti symmetric stretching vibration (vs) [7]

[8].

𝑣 (𝑐𝑚−1)

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(A)

Hasanain Saad Azeez

76

Fig.(4): Shows FTIR spectra for (A) K2CrO4 powder (B) K2Cr2O7 powder.

4. Conclusions 1. The X-ray diffraction (XRD) results showed

that K2CrO4 and K2Cr2O7 nanoparticles

powder have good crystallinity and in

general the grain size of K2CrO4 compound

was smaller than that of K2Cr2O7 compound.

2. The Atomic force microscopy (AFM)

results showed that K2CrO4 compound was

more average grain size and roughness than

that for K2Cr2O7 compound.

3. FTIR spectra show absorption bands

assigned both stretching and bending

vibrations related to the two compound

K2CrO4 and K2Cr2O7.

References

[1] Anger G., Halstenberg J., Hochgeschwender

K., Scherhag Ch., Korallus U., Knopf H.,

Schmidt P., Ohlinger M., “Chromium

Compounds”, in Ullmann's Encyclopedia

of Industrial Chemistry, Wiley - VCH,

Weinheim, 2005.

[2] Lewis R.J., “Hazardous Chemicals Desk

Reference”, Wiley-Interscience, 6th

Edition, 2008.

[3] Saha M., Srinivas C.R., Shenoy S.D.,

Balachandran C., Acharya S., “Footwear

dermatitis”, Contact Dermatitis, 28(5), 260–

264, 1993.

[4] Roto P., Sainio H., Reunala T., Laippala P.,

“Addition of ferrous sulfate to cement and

risk of chromium dermatitis among

construction workers”, Contact Dermatitis,

34(1),43–50, 1996.

[5] Herzberg G., “Infrared and Raman Spectra

of polyatomic molecules”, D. Vanhastrand

Co. Inc., New Yourk 1945.

[6] Chowdari B.V.R., Ravisekhar Y., “optical

properties of dichromate centers in some

lattices”, chemical physics letters, 59(2),

311-315, 1978.

[7] Stammreich H., Bassi D., Sala O., Siebert

H., “The vibrational spectrum of the

dichromate ion” 13, 192-198, 1958.

[8] Carter R. L., Bricker C. E., “vibrational spectrum of Triclinic potassium dichromate”, Spectroscopy letters, 2(8), 247-253, 1969.

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